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c
ʣ⇅10
4
, C/m
2
ʳ
m
a
b
ʣ⇅10
4
, C/m
2
ʣ⇅10
4
, C/m
2
ʳ
m
ʳ
m
Fig. 9.3
Effective surface charge density at the SW front versus strain amplitude in a monocrystal
NaCl. The plane SW propagated along the following crystallographic axes: (
a
) Œ1;0;0,(
b
) Œ1;1;0,
and (
c
) Œ1;1;1.The
circles
refer to experimental data (Mineev and Ivanov
1976
) while the theory
(Sirotkin and Surkov
1986
) is shown with
solid lines
Notice that the calculated multiplication coefficients of the dislocations, M
d
, and
of the point defects, M
p
, are close to the experimental data (Pierce
1961
) both for
the case of a dislocation mechanism of charge transfer and for the case of a point
defect. The strain threshold
for the point defects is found in the last column of
the table. As we have noted above, the mechanism of dislocations is more preferable
because their strain threshold is smaller by 2-3 orders of magnitude. However, it
should be noted that the parameters M
d
and M
p
are related as q
d
M
d
eM
p
.
Based on this fact one can suppose that point defects are captured and carried away
by dislocations; this means that such processes can define a value of dislocation
charge.
Table shows maximum values of electric field, E
, under the strain corresponding
to the points of sharp changes in the trends of Fig.
9.3
. The above analysis is not
valid in the region of high strains, when a change in polarity of the electric signal
is observed. What draws first attention is that the values of E
are lower by 1-2
orders of magnitude than the critical field, E
c
, of dielectric breakdown (e.g., E
c
1:3
10
8
V/m for NaCl). However, we should take into account that the increase
in defect number density in the SW and substantial deformation of the lattice under
0:3 can lead to the formation of additional local levels in the forbidden zone of
the crystal, i.e., to the decrease of E
c
. Thus one cannot exclude the effect of electron
breakdown of the crystal to explain the change of signal polarity at high pressure.
However this abrupt change of charge transfer mechanism in shock-compressed
ionic crystals has been something of a mystery.
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